Doctor blade arrangement and method in fibre processing

ABSTRACT

Apparatus for removing fiber webs from fiber conveying supports are disclosed including a doctor beam, a doctor blade having a web contacting edge, a hose for fluids for applying a force between the doctor beam and the doctor blade when pressurized to move the web contacting edge towards the fiber conveying support, the fiber web having a basis weight over 1000 g/m 2 , the doctor blade and doctor beam being pivotably attached, and the hose arranged for applying the force at a distance form the pivotable attachment to move the web contacting edge by pivoting the doctor blade relative to the doctor beam.

TECHNICAL FIELD

The present invention relates in general to a fibre dewatering press andin particular to arrangement for removing a fibre web from a wire in awire press.

BACKGROUND

Dewatering presses for dewatering of a fibre suspension and forming of acontinuous web thereof are previously known. One example of a knowndewatering press is a twin-wire press, see e.g. the publishedinternational patent application WO 2008/105706. Dewatering of the pulpis usually done from an inlet pulp concentration of 3-8% by weight to anoutput pulp concentration of 30-50% by weight. According to the state ofthe art, such twin-wire presses comprises two endless wires cooperatingfor dewatering fibre suspensions provided between them. The fibres forma continuous fibre web provided at an outlet from the twin-wire press.

When the continuous fibre web leaves the twin-wire press, the wires arefed back into the press while the continuous fibre web is supposed to bereleased from the wires. In order to ensure that parts of the fibre webdo not follow the wires back in to the twin-wire press, doctor bladesare typically provided at the surface of the wires at the outlet fromthe twin-wire press. It is desired that the doctor blade follows thesurface of the wire in close proximity when there is a potential riskfor fragments of the fibre web to follow the wire. However, the rolldriving the wire, which roll typically is made of a relatively softrubber material, is easily worn. After some operation time, the rollpresents a worn and uneven surface. Consequently, the wire that isformed according to the roll surface therefore also often exhibits anuneven surface, even if the wire itself is undamaged. A doctor bladeoriginally mounted in close proximity to the original wire surface isthereby separated from at least parts of the wire surface. Also, if thedoctor blade is pushed against such an uneven surface, the doctor blademay damage the raised parts of the wire. The operating conditions atthis position are also relatively demanding. The temperature is often inthe range of 70-90° C. and may vary with time and the environmenttypically comprises peroxides. In order to reduce the wear of the wireand at the same time withstand the demanding conditions, the doctorblade is typically made of a polymer material, typically ahigh-molecular polythene material. Furthermore, at these temperatures,plastic deformation of the material is not unusual as well asdisplacements resulting from changing temperatures in different parts ofthe press.

A general problem in prior art is that the doctor blade arrangementscause too much wear on the wire and/or cannot compensate for worn wires,doctor blade deformation or shifting surrounding temperatures.

RELATED ART

Doctor blades as such are also utilized e.g. in connection with papermachines, such as e.g. disclosed in the U.S. Pat. No. 1,566,358 or2,914,788. However, the fibre web in a paper machine is considerablylighter than for pulp applications. In pulp applications, the basisweight is typically higher than 1000 g/m². Furthermore, also otherconditions differ considerable between pulp and paper applications.Fibre webs of different weights and at different conditions will behavevery differently and doctor blade solutions found in paper manufacturingapplications cannot without careful modifications be utilized for pulpmanufacturing purposes.

SUMMARY

An object of the present invention is therefore to provide a doctorblade arrangement presenting low wear on the wire, well adaptedresilience behaviour and suitable scraping properties.

The above objects are achieved by arrangements and methods according tothe enclosed patent claims. In general words, in a first aspect, anarrangement for removal of a fibre web from a fibre conveying supportcomprises a doctor beam and a doctor blade having a web contacting edge.The fibre web has a basis weight over 1000 g/m². The doctor blade andthe doctor beam are attached to each other by a pivotable attachment.The arrangement further comprises a hose for fluids arranged forapplying a force between the doctor beam and the doctor blade at adistance from the pivotable attachment when being pressurized to movethe web contacting edge towards the fibre conveying support by pivotingthe doctor blade relative to the doctor beam.

In a second aspect, a twin-wire press for dewatering of a fibresuspension, comprising lower rolls, an endless lower wire, upper rollsand an endless upper wire, and an arrangement according to the firstaspect arranged for removal of a fibre web from at least one of theendless upper wire and the endless lower wire.

In a third aspect, a method for operating an arrangement for removal ofa fibre web from a fibre conveying support comprises pressurizing of ahose for fluids arranged for applying a force between a doctor beam anda doctor blade at a distance from a pivotable attachment between thedoctor beam and the doctor blade to move a web contacting edge of thedoctor blade towards the fibre conveying support by pivoting the doctorblade relative to the doctor beam. The method further comprisescontrolling of the pressurizing dependent on an operating condition of adevice providing the fibre web.

One advantage with the present invention is that the wear on doctorblades and fibre conveying supports are significantly lowered.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by making reference to the following descriptiontaken together with the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a twin-wire press;

FIG. 2 is a schematic illustration of an embodiment of a doctorarrangement according to the present invention;

FIG. 3 is a schematic illustration of the embodiment of FIG. 2 withpressurized hose;

FIG. 4 is a block diagram of an embodiment of a twin-wire pressarrangement according to the present invention;

FIG. 5 is a flow diagram of steps of an embodiment of a method accordingto the present invention; and

FIG. 6 is a schematic illustration of another embodiment of a doctorarrangement according to the present invention.

DETAILED DESCRIPTION

Throughout the drawings, the same reference numbers are used for similaror corresponding elements.

FIG. 1 illustrates schematically a twin-wire press 1. The illustrationis very simplified in order to facilitate the understanding of thegeneral principle. The twin-wire press 1 comprises an endless upper wire6A running in a path around upper rolls 8A. The twin-wire press 1 alsocomprises an endless lower wire 6B running in a path around lower rolls8B. A fibre suspension is entered into the space between the endlessupper wire 6A and the endless lower wire 6B through a headbox 10 at aninlet 2 of the twin-wire press 1. The flow path of fibre suspension/webis indicated by the arrows 5. The fibre suspension is thus provided intoa space 14 between the endless upper wire 6A and the endless lower wire6B. The two wires 6A, 6B cooperate with each other through a dewateringsection 20 of the path 5, in which the wires 6A, 6B form a wedge-shapeddewatering space for the fibre suspension between each other. Duringdisplacement of the wires 6A, 6B through the dewatering section 20, thewires 6A, 6B thus successively compress the fibre suspension in thewedge-shaped space, whereby the fibre suspension is initially pressedand dewatered and formed to a continuous fibre web between the wires 6A,6B. At an outlet 3 from the twin-wire press 1, the fibre web is releasedfrom the wires 6A, 6B and collected in a shredder 12. Doctorarrangements 30 are provided at the surface of the wires 6A, 6B at arespective roll 8A, 8B.

At the outlet from the twin-wire press 1, the fibre web is peeled offfrom the wires. In most situations, the fibre web is uniting in itselfand the fibre web is typically released from the wires without problems.However, at some occasions, e.g. when starting or stopping the twin-wirepress or when disturbances normal operation conditions appear, theremight be portions (or even the entire web) which continue to follow oneof the wire surfaces instead of being released. If such fibre materialportions are allowed to follow the endless wire into the interior of thetwin-wire press again, this can cause serious problems. A doctorarrangement 30 is therefore typically provided in connection with theoutlet 3 from the twin-wire press 1. As described in the backgroundsection, prior art doctor arrangements have a number of drawbacks. Thedoctor arrangement 30 is provided at least one of the wires 6A, 6B, andtypically at both.

One embodiment of an arrangement for removal of a fibre web from a wireaccording to the present invention is illustrated in FIG. 2, i.e. adoctor arrangement 30. A roll 8 drives a fibre conveying support 21, inthis embodiment an endless wire 6, in a conveying direction 26. In otherembodiments, the fibre conveying support 21 could e.g. be the surface ofa roll. A fibre web, in this embodiment a web of cellulose pulp, isintended to be conveyed on a surface 24 of the endless wire 6 and bereleased therefrom before the endless wire 6 returns into the interiorof the twin-wire press. The doctor arrangement 30 comprises a doctorblade 32 attached to a doctor beam 34. The doctor blade 32 is providedfor prohibiting any parts of the fibre web to follow the fibre conveyingsupport beyond the outlet 3. The doctor blade 32 is mechanicallysupported by the doctor beam 34, and the doctor beam 34 constitutes theattachment of the doctor blade 32 to the main fibre handling equipment,i.e. in this embodiment the twin wire press. In this embodiment, aportion of the doctor beam is formed as a clamping plate 36, whichclamps the doctor blade 32 against the doctor beam 34 with a pivotingpoint 35. The clamping plate 36 is firmly attached to the main part ofthe doctor beam 34, in this embodiment by bolts 38. The clamping plate36 is in the present embodiment bent in such a way that outside thepivoting point 35, there is a distance 39 between the doctor blade 32and the clamping plate 36, which allows the doctor blade to pivot outfrom the doctor beam surface around the pivoting point 35. The clampingplate 36 and the end of the doctor beam 34 thus together form apivotable attachment 37 of the doctor blade 32.

The doctor arrangement 30 is preferably originally mounted in such a waythat a web contacting edge 33 of the doctor blade 32 is placed just atthe surface 24 of the wire 6, when the doctor blade 32 is positionedalong the surface of the doctor beam 34. When surrounding conditions arechanged, e.g. due to wear of the roll 8 or wire 6, differenttemperatures or plastic deformation of the doctor blade, a slit may beleft between the web contacting edge 33 and the surface 24 of the wire.For such occasions, a hose 40 for fluids is provided in a recess 41 ofthe doctor blade 32. The hose 40 can generally be pressurized with anyfluid-gas or liquid and is therefore adapted for being connected to ahose pressurizing arrangement. In the present embodiment, the hose 40 isintended to be pressurized by air.

FIG. 3 illustrates a situation when the hose 40 is pressurized by a (notshown) hose pressurizing arrangement. The hose 40 expands and protrudesoutside the recess 41 and applies a force onto a support surface 42 ofthe doctor beam 34. This support surface 42 can be a portion of the maindoctor beam 34 itself or be a portion provided with a surface coatingadapted for interaction with the expanded hose 40. In general words, thehose 40 is arranged for applying a force between the doctor beam 34 andthe doctor blade 32 at a distance from the pivoting point 35 when beingpressurized. The rear end of the doctor blade 32 is thereby moved outfrom the surface of the doctor beam 34, leaving a space 43 therebetween. This force acts to move the web contacting edge 33 towards thefibre conveying means 21, in this embodiment the wire 6. The webcontacting edge 33 is thereby held against the surface 24 of the wire 6with a force that is determined by the pressure applied in the hose 40.The pivoting of the doctor blade 32 is typically limited by the distance39 to the clamping plate 36. The web contacting edge 33 follows thesurface 24 of the wire 6 even if the roll or wire is unevenly worn. Alsomechanical movements in the doctor arrangement 30 as a result of e.g.temperature differences are compensated by such an arrangement.Furthermore, if the doctor blade 32 itself, which typically is made ofhigh-molecular polythene, undergoes plastic deformation, also this iscompensated. The doctor blade 32 is preferably manufactured in amaterial that is relatively soft, such as e.g. high-molecular polythene,which allows the doctor blade 32 to adapt and/or be worn according tothe surface profile of the wire and/or roll.

Now returning to FIG. 2. When the rather thick fibre web comes intocontact with the doctor blade 32 that is gently pressed against the wire6, the fibre web also creates a force F on the doctor blade 32. Thisforce F increases the pressure on the web contacting edge 33 against thesurface 24 of the wire 6. The doctor blade 32 wants to rotate around thepivot point 35. A higher normal force N is thereby obtained, whichincreases the friction force R from the wire 6. The higher the force Fbecomes the better and tighter the doctor blade 32 is pressed againstthe wire 6, and the doctor blade 6 doctors away the fibre web from thewire. Because of this function, there is only need of a small pressurein the hose 40 at normal running to keep the doctor blade 32 close tothe wire 6. Such a lower pressure gives a lower friction and longerlifetime of the wire 6 and doctor blade 32.

Thus, during the majority of the operation time of a twin-wire press,the release of the fibre web, i.e. in this case the release of a pulpmat, functions without any need for assistance from any fluid loadeddoctor arrangements. During such periods, it would therefore bebeneficial if the doctor blade 32 is not actively pressed against thewire 6 surface. As mentioned above, this could reduce the wear both onthe wire 6 and/or roll 8 and on the doctor blade 32. There is thereby aneed for being able to control the pressurizing of the hose 40 dependingon the actual operating conditions of the twin-wire press.

The upper doctor blade of a twin-wire press is typically in no contactwith the pulp mat when the machine is running. The doctor blade onlypicks up or pulls out small amount of fibres that is stuck into thewire. The smaller fibre particles more or less follow the wire, even ifthe doctor blade has loosened the fibre from the wire. Spraying systemsare typically provided to handle these particles later.

Also the lower doctor blade is in no contact to the pulp mat when themachine is running properly. The pulp mat structure and its own weightis pulling the pulp mat out from the wire so that the doctor blade goesfree without contact to the pulp mat.

This typical phenomenon occurs in particular with pulp mats that has asufficient strength and weight, and it is presently believed that it isa requirement to have a basis weight that is over 1000 g/m², andpreferably over 1200 g/m², to achieve this phenomenon. In start upprocesses and shut down processes of the machine, often both the upperand lower doctor blades are in contact with the pulp mat. Then there isa need to have a strong doctor blade, but made of a gently material forreducing the damage on the wire. This is because the high contact forcesthat this thick pulp mat generates. The minimum basis weight for pulpmats produced by twin-wire presses is 1200 g/m², which makes the presentdoctor blade arrangements according to the present inventionparticularly suitable.

A further advantageous function for the lower doctor blade arrangementis to provide for a contact between the pulp mat and the clamping plate.This stabilizes the pulp mat on its way to the shredder screw. Withoutthis contact, the pulp mat is more easily broken because of the actionof the forces that are applied at the shredder screw and from the weightitself. When a break occurs, the pulp mat looses the force from its ownweight that pulls out the pulp mat and there is typically a doctor bladecontact against this very thick pulp mat that gives more fibre lossesand wearing on the doctor blade. Typically, the speed of the wire is inthe range of 10-40 m/min at normal operation.

A fibre handling system according to an embodiment of the presentinvention is illustrated in FIG. 4. A twin-wire press 1 for dewateringof a fibre suspension has doctor arrangements 30, e.g. according to theembodiment of FIGS. 2 and 3, mounted at the outlet from the twin-wirepress 1. A hose pressurizing arrangement 50 is connected to a hose ofthe doctor arrangements 30. A controller 52 is arranged for controllingthe hose pressurizing arrangement 50. The controller 52 is connected forreceiving information about the operation conditions of the twin-wirepress 1. Based on this information, the controller 52 can pressurize thehoses of the doctor arrangements 30 when needed, e.g. during starting orstopping of the twin-wire press 1, or when other disturbances in theoperation is detected. During normal operation of the twin-wire press 1,the controller 52 can instead allow a release of the pressure andthereby allow the doctor blade to pivot out from the wire surface.

FIG. 5 illustrates a flow diagram of steps of an embodiment of a methodaccording to the present invention. A method for operating anarrangement for removal of a fibre web from a wire starts in step 200.In step 210, a hose for fluids is pressurized. The hose is arranged forapplying a force between a doctor beam and a doctor blade at a distancefrom a pivotable attachment between the doctor beam and the doctor bladeto move a web contacting edge of the doctor blade towards the wire. Thepressurizing is in step 212 controlled dependent on an operatingcondition of a device providing the fibre web. The procedure ends instep 299.

The embodiment illustrated in FIGS. 2 and 3 has the hose provided in arecess in the doctor blade and arranged to actuate on the doctor beam.Furthermore, the hose applies the force on the doctor blade at a sideopposite to the web contacting edge relative to the pivotable attachment37 and pivoting point 35. However, there are also alternativeembodiments. One alternative embodiment is illustrated in FIG. 6. Here,the hose 40 is provided in a recess 41 in the doctor beam 34 instead.The hose 40 is thereby arranged to actuate on a support surface 42 atthe doctor blade 32. In another alternative embodiment the hose 40 canbe provided such that the hose applies the force on the doctor blade ata same side as the web contacting edge relative to the pivotableattachment.

The clamping of the doctor blade 32 to the doctor beam 34 at limitedareas gives rise to a simple embodiment of a pivotable attachment.Anyone skilled in the art realizes that also other types of pivotableattachments can be utilized, such as different kinds of hinges.

In the embodiments above, the fibre conveying support is exemplified bya wire. However, the present ideas also operate well with also othertypes of fibre conveying support, such as e.g. different kinds of rolls.

The conditions at the outlet from a fibre web handling arrangement, suchas a twin-wire press, are relatively special. The environment ishazardous, typically comprising peroxides. At the same time, thetemperatures are often in the range of 70-90° C., but may also varyconsiderably, in particular at starting and stopping the fibre webhandling arrangement. The presented solutions are well adapted towithstand such environments at the same time as they provide for simpleand cost-efficient operation.

The embodiments described above are to be understood as a fewillustrative examples of the present invention. It will be understood bythose skilled in the art that various modifications, combinations andchanges may be made to the embodiments without departing from the scopeof the present invention. In particular, different part solutions in thedifferent embodiments can be combined in other configurations, wheretechnically possible. The scope of the present invention is, however,defined by the appended claims.

1. An arrangement for removal of a fiber web from a fiber conveyingsupport, comprising: a doctor beam; a doctor blade having a webcontacting edge; and a hose for fluids arranged for applying a forcebetween said doctor beam and said doctor blade when being pressurized tomove said web contacting edge towards said fiber conveying support; saidfiber web having a basis weight over 1000 g/m²; said doctor blade andsaid doctor beam being attached to each other by a pivotable attachment;and said hose being arranged for applying said force at a distance fromsaid pivotable attachment to move said web contacting edge by pivotingsaid doctor blade relative to said doctor beam.
 2. The arrangementaccording to claim 1, wherein said hose is provided in a recess in saiddoctor blade and arranged to actuate on said doctor beam.
 3. Thearrangement according to claim 1, wherein said hose is provided in arecess in said doctor beam and arranged to actuate on said doctor blade.4. The arrangement according to claim 1, wherein said hose is applyingsaid force on said doctor blade at a side opposite to said webcontacting edge relative to said pivotable attachment.
 5. Thearrangement according to claim 1, wherein said hose is a gas hose. 6.The arrangement according to claim 1, wherein said pivotable attachmentis provided by clamping said doctor blade between a main portion of saiddoctor beam and a clamping plate, in turn mechanically attached to saidmain portion of said doctor beam.
 7. The arrangement according to anyclaim 1, further comprising a hose pressurizing arrangement connected tosaid hose.
 8. The arrangement according to claim 7, further comprising acontroller arranged for controlling said hose pressurizing arrangement.9. A twin-wire press for dewatering of a fiber suspension, comprisinglower rolls, an endless lower wire, upper rolls and, an endless upperwire, and an arrangement for removal of a fiber web from at least one ofsaid endless upper wire and said endless lower wire, said arrangementfor removal of a fiber web in turn comprising: a doctor beam; a doctorblade having a web contacting edge; and a hose for fluids arranged forapplying a force between said doctor beam and said doctor blade whenbeing pressurized to move said web contacting edge towards said fiberconveying support; said fiber web has a basis weight over 1000 g/m²;said doctor blade and said doctor beam being attached to each other by apivotable attachment; and said hose being arranged for applying saidforce at a distance from said pivotable attachment to move said webcontacting edge by pivoting said doctor blade relative to said doctorbeam.
 10. A method for operating an arrangement for removal of a fiberweb from a fiber conveying support, comprising the steps of:pressurizing a hose for fluids arranged for applying a force between adoctor beam and a doctor blade to move a web contacting edge of saiddoctor blade towards said fiber conveying support; pivoting said doctorblade relative to said doctor beam by applying said force at a distancefrom a pivotable attachment between said doctor beam and said doctorblade to provide said moving of said web contacting edge; andcontrolling said pressurizing dependent on an operating condition of adevice providing said fiber web.
 11. The arrangement according to claim2, wherein said hose is applying said force on said doctor blade at aside opposite to said web contacting edge relative to said pivotableattachment.
 12. The arrangement according to claim 3, wherein said hoseis applying said force on said doctor blade at a side opposite to saidweb contacting edge relative to said pivotable attachment.
 13. Thearrangement according to claim 2, wherein said hose is a gas hose. 14.The arrangement according to claim 3, wherein said hose is a gas hose.15. The arrangement according to claim 4, wherein said hose is a gashose.
 16. The twin-wire press according to claim 9, wherein said hose isprovided in a recess in said doctor blade and arranged to actuate onsaid doctor beam.
 17. The twin-wire press according to claim 9, whereinsaid hose is provided in a recess in said doctor beam and arranged toactuate on said doctor blade.
 18. The arrangement according to claim 9,wherein said hose is applying said force on said doctor blade at a sideopposite to said web contacting edge relative to said pivotableattachment.
 19. The arrangement according to claim 16, wherein said hoseis applying said force on said doctor blade at a side opposite to saidweb contacting edge relative to said pivotable attachment.
 20. Thearrangement according to claim 17, wherein said hose is applying saidforce on said doctor blade at a side opposite to said web contactingedge relative to said pivotable attachment.